I am analyzing the permeability of continuous and fenestrated capillaries to large molecules, and I utilize for the analysis electron microscopy and its ancillary techniques. My main approach is to introduce into the circulation tracer molecules detectable by electron microscopy and follow their passage across the endothelium into the pericapillary space. As tracers I use ferritin and peroxidatic enzymes of different molecular sizes. I am examining the role of endothelial fenestrae, micropinocytic vesicles and junctions in transfer of the large molecules. I shall also undertake studies to determine how lymphatics take up proteins from the surrounding interstitium and to determine whether proteins pass, via vesicles, from the interstitium across the endothelium into the capillary lumen. I have been studying the structure of endothelial junctions by the freeze-fracture technique. Another worker in the laboratory is studying the distribution and ultrastructure of small, intensely fluorescent neurons of the nodose ganglion.